Nick Clegg has accused First Minister Alex Salmond and Ukip leader Nigel Farage of a "politics of fear" that offers false comfort but cannot improve voters lives.
The LibDems' leader used a combative conference speech, his last before the General Election, to also launch staunch attacks on the Tories and Labour.
The Conservatives were guilty of a lurch to the right in response to Ukip, while Labour was still putting its head in the sand over the nation's finances, he said.
In a speech light on policy announcements, he did signal one possible 'red line' in any future Coalition talks.
He told delegates Theresa May was "playing politics" with national security and pledged he would not agree to her planned snoopers' charter. In a direct message to the Home Secretary, he said: "We blocked it once and we will do it again".
With polls suggesting just 7 per cent of voters plan to back the Liberal Democrats next May, his main pitch was the party are the viable alternative to two unappealing choices at Westminster.
But he did not pull his punches on the SNP and UKIP.
He told delegates at the annual conference in Glasgow that life is "simple when you know who - or what - to blame. It's seductive and it's beguiling."
He accused the SNP of blaming "the English" for Scotland's woes.
"But resentment, the politics of fear, doesn't pay the bills or create a single job. Claiming to address people's acute anxiety about the modern world, it provides nothing but the false comfort of grievance. Dressed up as the politics of hope, it is, in fact, a counsel of despair."
Unlike the Conservatives' leader David Cameron last week, Mr Clegg did not use the speech to set out a series of election promises.
The LibDems are cautious amid concerns the public does not believe them following their U-turn on tuition fees in 2010.
He admitted it had been difficult going into conference with a message centred not on the LibDems themselves but on their differences with their two main rivals at Westminster.
But he added that Labour and the Tories had helped in the last three weeks, with Conservative plans to squeeze working age benefits and the Labour leader forgetting to mention the deficit in his conference speech.
"So thank you Ed Miliband and thank you George Osborne."
In a sign of the further deterioration of Coalition relations, he also revealed details of a private conversation with Mr Osborne.
Mr Clegg claimed the Chancellor told him before 2012 that he had to cut taxes for the wealthy because he could not afford politically to deliver a "LibDem budget".
Somebody had to stand up for the "decent liberal" Britain in which millions still believe, he said.
Mr Clegg also pleaded with voters not to judge the party solely on tuition fees, but on its successes in Government, including on raising the personal income tax threshold, effectively giving a tax cut to millions.
Mr Clegg said his message was "opportunity for everyone". He said that with this message "the anger will wane" and the hope of a better future can take root.
In a heavily trailed passage in the speech he also criticised David Cameron's response to the independence referendum result.
An SNP spokeswoman said: "Nick Clegg should reflect on the fact the LibDems were on the same side as Ukip in the independence referendum.
"And this week the LibDems sold out their last remaining principle, admitting they are willing to back the Tory in/out referendum on Europe.
"The LibDems are prepared to do anything to prop up their Tory allies and dance to Ukip's Euro-hostile tune, which is why they are at 5% in the polls in Scotland and the SNP lead Labour by 15 points."
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